In the field of separating out solid particles in gaseous suspension it is customary to use cyclones as the mechanism for separating out the suspended solids. Such cyclones normally have a valve at their ends where the solid material is discharged or, as is more well-known, at the ends of the legs of the cyclones. This valve acts as a sealing element, preventing a gaseous counterflow current towards the inside of the cyclone, which would considerably reduce its separation efficiency. On the other hand, however well the valve is designed, it is mechanically impossible for a single valve to eliminate completely the draw-back of solid particles due to the counterflow of the gaseous current in the interval between the opening and subsequent closing of the valve.
In the specific case of fluid catalytic cracking (FCC) processes, it is known that the internal pressure in the cyclone is always below the pressure of the separating vessel surrounding it, which requires the use of sealing valves at the lower end of the legs of the cyclones.
Next, a critical point, which directly influences the efficiency of the FCC process, is the seal provided by the sealing valves in the cyclone legs. As they have to operate under fairly harsh conditions, such as temperatures in excess of 500.degree. C. and a solids charge rate of the order of 20-50 t/min, these valves do not normally provide a perfect seal, so they permit to some extent the passage of gas from the separating vessel towards the inside of the leg of the cyclone. A flow in excess of 0.5% of the total volume fed into the inlet port of the cyclone causes draw-back of the fine solid particles, which disrupts the operation of the cyclone and thereby reduces its separation efficiency.
As an alternative solution, GB-A-2,272,248 proposes the use of a deadweight valve which provides a virtually total seal through the pressure of a conical section installed between the base of the valve and the seat of the leg of the cyclone.
However, it is important to remember that a small amount of gas passing through the sealing valve, of the order of 0.01 to 0.5% of the total volume fed into the port of the cyclone, is beneficial since it causes fluidification of the solid which will have accumulated in the leg of the cyclone, which promotes its discharge.
In this way, an attempt to prevent the passage of the gas flow through the valve, as proposed by the aforementioned patent, may give rise to the loss of fluidification of the solid which has accumulated in the leg of the cyclone, principally in the second phase of separation when the charge of solids is fairly dilute, e.g. of the order of 0.2 to 7.5 grams of particulate per cubic meter of gas. Under such operating conditions the accumulation of solids, leading to a balance in pressure and resulting discharge of the solids, may require a period in excess of eight hours to effect adequate build-up and, if there is not a minimum flow of gas through the valve, the dense bed of particulates may become "packed down", with the consequent risk of complete obstruction of the leg of the cyclone and a drop in the efficiency of the cyclone.